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RE: Nested Aggregate Expressions

From: Polleres, Axel <axel.polleres@siemens.com>
Date: Tue, 5 Jun 2012 17:06:59 +0200
To: "birte.glimm@uni-ulm.de" <birte.glimm@uni-ulm.de>, "andy.seaborne@epimorphics.com" <andy.seaborne@epimorphics.com>
CC: "public-rdf-dawg@w3.org" <public-rdf-dawg@w3.org>
Message-ID: <9DA51FFE5E84464082D7A089342DEEE8013A3A9678D7@ATVIES9917WMSX.ww300.siemens.net>
FWIW, on a quick strawpoll on today's telco on the issue we got

 4 x +1 to disallow nested aggregates
   and
 2 x 0 (not sure/not swapped in)
 1 x 0 no preference either way

Best,
Axel

--
Dr. Axel Polleres
Siemens AG Österreich
Corporate Technology Central Eastern Europe Research & Technologies
CT T CEE

Tel.: +43 (0) 51707-36983
Mobile: +43 (0) 664 88550859
Fax: +43 (0) 51707-56682 mailto:axel.polleres@siemens.com


> -----Original Message-----
> From: Birte Glimm [mailto:birte.glimm@uni-ulm.de]
> Sent: Tuesday, 5 June 2012 5:00 PM
> To: Andy Seaborne
> Cc: public-rdf-dawg@w3.org
> Subject: Re: Nested Aggregate Expressions
>
> [snip]
> >>> One line of argument is that the expression inside the
> aggregate is
> >>> applied to each row, so only row variables should be
> considered in-scope.
> >>>  The aggregate AVG(max(?x)+1) is violating that as
> max(?x) is not a
> >>> per-row expression.
> >
> > (Birte) - yes this needs clarifying if we wish to rule it out, and
> > possible even if we don't.
> >
> > As the spec stands, I *think* it says its not allowed:
> >
> > [[
> > Definition Group:
> >
> > Group evaluates a list of expressions against a solution
> sequence ...
> > ]]
> >
> > and the solution sequence is the grouped patterns, not after
> > aggregation or select expressions.
> >
> > [[
> > Definition: Aggregation
> > ]]
> > talks about applying the aggregate to the solution
> sequences collected
> > into a map of key to multiset.
> >
> > i.e. the aggregate is evaluated over the pattern, not other
> aggregates
> > and not select expressions
>
> I think the definition just cannot handle the current case,
> but it is not forbidden, just undefined. IMO, either the
> definition has to be extended or the current case has to be
> forbidden. Maybe it is illegal due to some hidden
> constraints, but that should be made exlicit.
>
> Birte
>
> > Steve - opinion?
> >
> >
> >
> >>>
> >>>
> >>> What ARQ does is to calculate the aggregates of a group
> as the group
> >>> streams past; it does not wait until the end of evaluation of the
> >>> whole block when all the elements of all the groups are known.
> >>>
> >>>
> >>> Related to this is the interaction with select expressions:
> >>>
> >>> SELECT (max(?x) As ?M) (avg(?M+1) AS ?A)
> >>>
> >>> because the select expression rules say you can use ?M
> inside AVG().
> >>>
> >>> If we wish to forbid this, we can do it quite easily by having a
> >>> parser rule that aggregates can't appear in expression for the
> >>> aggregate, which is a simple static check.
> >>
> >>
> >> Oh boy, it's certainly wacky.
> >>
> >> That parse rule wouldn't rule out the use of ?M above
> though anyway,
> >> would it?
> >
> >
> > Complicated :-)
> >
> > As I read the spec, the ?Ms are different.
> >
> > (max(?x) As ?M) -- select expression
> >
> > avg(?M+1) -- undefined variable in the grouped pattern that
> is never
> > mentioned or bound.
> >
> > Like writing
> >
> > avg(?noSuchVariable+1)
> >
> > -----
> >
> > Turning this round:
> >
> > Does any one have a use case that suggests it should be legal?
> >
> >        Andy
> >
> >>
> >> - Steve
> >>
> >
>
>
>
> --
> Jun. Prof. Dr. Birte Glimm            Tel.:    +49 731 50
> 24125 Inst. of Artificial Intelligence         Secr:  +49 731
> 50 24258 University of Ulm                         Fax:   +49
> 731 50 24188
> D-89069 Ulm
> birte.glimm@uni-ulm.de Germany
>
>
Received on Tuesday, 5 June 2012 15:07:31 GMT

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